This invention relates to a DC-DC conversion type power supply unit having soft start functionality, and to a portable apparatus equipped with such power supply unit.
Conventional portable apparatuses such as PDAs and personal computers (PCs) have a multiplicity of DC-DC conversion type power supply circuits one for each component such as a display, a communications unit, and a control unit (e.g. CPU), and a common battery to supply electric power to the respective power supply circuits. Each of the power supply circuits is formed in the respective component of a semiconductor device so as to generate a predetermined voltage for the device. In most cases, the power supply circuit is configured to become operable only when the output of the device is needed. In this way, the operable time of the common battery can be extended by saving its energy when it is not needed by the power supply circuit.
These power supply circuits are constructed in the forms of series-pass type DC power sources, step-up type switching DC power sources, and step-down type switching DC power sources. In either of these DC power supply circuits, a large current flows through it during a startup of the circuit, which results in a large drop, and hence an instability, in the output voltage thereof (referred to as source voltage). Thus, fluctuations of the source voltage take place every time one of the power supply circuits is started up, thereby resulting in adverse effects including malfunctions and/or erroneous operations of the control circuits of a semiconductor device if it is equipped with these power supply circuits. The fluctuations of the source voltage can be suppressed by providing the power supplying battery with a sufficient capacity to absorb a large current change. However, it is difficult to do so in portable apparatuses, since they must be compact in size and light in weight.
In order to circumvent such adverse effects, during a startup of a power supply circuit, a so-called soft start is recommended in which the output voltage of a power supply circuit is slowly increased. To do so, generally, the reference voltage supplied to the power supply circuit is gradually increased from zero.
In a case where the semiconductor device includes only one power supply circuit, there will be no problem in the gradual increase of the reference voltage in the soft start. However, this is not the case if the semiconductor device includes multiple power supply circuits that should be individually actuated and stopped. In such a case as mentioned above, in order to allow for a soft start of any power supply circuit, the reference voltage is varied every time one of the power supply circuits is actuated. As a consequence, the rest of the power supply circuits are affected by the change in the reference voltage.
One way to avoid such unintended fluctuations of the reference voltage is to employ multiple voltage sources providing only reference voltages (referred to as reference-voltage sources) to thereby allow for individual soft starts of the respective power supply circuits. However, provision of multiple reference-voltage sources inevitably results in an increase in the area of the voltage sources in the semiconductor device. In addition, such multiple voltage sources disadvantageously consume large power.
It is, therefore, an object of the present invention to provide a DC-DC conversion type power supply unit capable of permitting soft starts of multiple power supply circuits such that they can be individually actuated and stopped while keeping constant (i.e. without varying) the reference voltage of a common reference-voltage source, thereby suppressing fluctuations of the output voltages of the power supply circuits.
It is another object of the invention to provide a portable apparatus equipped with such power supply unit.
In accordance with one aspect of the invention, there is provided a DC-DC conversion type power supply unit for outputting a predetermined output voltage by converting a given source voltage of a DC power source, comprising:
a reference-voltage source;
an error amplifier including
a first input terminal for receiving a reference voltage from said reference-voltage source,
a second input terminal for receiving as a feed back voltage a voltage associated with the output voltage,
an output terminal, and
a feedback circuit that has a condenser (i.e. capacitor) connected between the second input terminal and the output terminal, the error amplifier adapted to amplify the voltage difference between the first and second input terminals to generate at the output terminal a control signal for controlling said output voltage; and
a voltage adjuster capable of setting the voltage of the second input terminal to a prohibition voltage that prohibits the predetermined output voltage from being outputted from the unit, wherein
the voltage adjuster is driven to fix the voltage of the second input terminal at the prohibition voltage for a predetermined period of time during a startup of the power supply unit.
In accordance with another aspect of the invention, there is provided a DC-DC conversion type power supply unit including a common reference-voltage source and a multiplicity of individually controllable DC-DC conversion type power supply circuits for converting the source voltage of a DC power source to a multiplicity of predetermined output voltages, wherein
each of the multiplicity of DC-DC conversion type power supply circuits comprises
an error amplifier including
a first input terminal for receiving a reference voltage from the reference-voltage source,
a second input terminal for receiving a voltage associated with the output voltage as a feed back voltage,
an output terminal, and
a feedback circuit that has a condenser connected between the second input terminal and the output terminal, the error amplifier adapted to amplify the voltage difference between the first and second input terminals to generate at the output terminal a control signal for controlling the output voltage; and
a voltage adjuster capable of setting the voltage of the second input terminal to a prohibition voltage that prohibits the predetermined output voltage from being outputted from the power supply circuit, and wherein
the voltage adjuster is driven to fix the voltage of the second input terminal to the prohibition voltage for a predetermined period of time during a startup of the power supply circuit.
In accordance with a further aspect of the invention, there is provided a portable apparatus equipped with a battery, a common reference-voltage source powered by the battery and a multiplicity of individually controllable DC-DC conversion type power supply circuits for converting the voltage of the battery to a multiplicity of predetermined output voltages, wherein
each of the multiplicity of DC-DC conversion type power supply circuits comprises
an error amplifier including
a first input terminal for receiving a reference voltage from the reference-voltage source,
a second input terminal for receiving the output voltage as a feed back voltage,
an output terminal, and
a feedback circuit that has a condenser connected between the second input terminal and the output terminal, the error amplifier adapted to amplify the voltage difference between the first and second input terminals to generate a control signal for controlling the output voltage provided at the output terminal; and
a voltage adjuster capable of setting the voltage of the second input terminal to a prohibition voltage that prohibits the predetermined output voltage from being outputted from the output terminal, wherein
the voltage adjuster is driven to fix the voltage of the second input terminal to the prohibition voltage for a predetermined period of time during a startup of the DC-DC conversion type power supply circuit.
In accordance with a further aspect of the invention, there is provided a method of starting up a DC-DC conversion type power supply unit for converting a source voltage of a DC power source to a predetermined voltage, comprising steps of:
inputting a reference voltage to a first input terminal of an error amplifier;
feeding back a voltage associated with the output voltage to a second input terminal of the error amplifier;
connecting a feedback condenser between the first input terminal and the output terminal of the error amplifier;
rendering the DC-DC conversion type power supply unit operable; and
fixing the voltage of the second input terminal at a prohibition voltage that prohibits the output voltage from being outputted from the power supply unit for a predetermined period of time during a startup of the DC-DC conversion type power supply unit to charge up the feedback condenser to a predetermined voltage; and
after the predetermined period of time, gradually varying the output of the error amplifier while discharging the charging voltage of the feedback condenser.
The invention enables soft start of a power supply unit by simply coupling the second input terminal of the error amplifier to a specific voltage without changing the level of the reference voltage at all. It will be appreciated that no soft start circuit is necessary for continuously changing the reference voltage to implement soft start of the power supply unit.
It is noted that in accordance with the invention a common reference-voltage source may be connected to a multiplicity of DC-DC conversion type power supply circuits such that they can be individually actuated and stopped without changing the reference voltage. As a consequence, any one of the power supply circuits can be started up without appreciably affecting a common power supply such as a battery. Thus, the invention provides a power supply unit suitable for miniaturized, light weight, battery-operated portable apparatuses such as cellular phones, PDAs, and PCs operating at low power.